The present invention relates to a color image forming apparatus on a non-contact developing system.
During the developing process in an image forming apparatus using electrophotography, a photoreceptor such as a photosensitive drum or a photosensitive belt is rotated at a fixed speed in a predetermined direction first and uniformly charged by a charging unit disposed around the photoreceptor. Then the photoreceptor is exposed to a light beam from an exposure unit and scanned, whereby an electrostatic latent image is formed on the photoreceptor. Further, the electrostatic latent image formed on the photoreceptor is developed by developing units (developing devices) disposed around the photoreceptor.
The developing unit is provided with a developing roller (toner carrier) for carrying toner to a developing position opposite to the photoreceptor. The toner from the developing roller is made to stick to the photoreceptor, so that the electrostatic latent image is reduced to a visible image. In the case of a color image forming apparatus, a series of charging, exposure and developing steps is successively repeated on a color basis (e.g., Y (yellow), M (magenta), C (cyan) and K (black)).
A related-art color image forming apparatus is provided with developing rollers for carrying four colors of YMCK in the respective developing units and when these colors are developed, there has been employed a system of moving the developing roller for an applicable color up to a predetermined position to make the developing roller contact or adjoin a photoreceptor. In other words, the operation of moving the developing roller has been needed at each step of developing the applicable color. In case where four color developing rollers are disposed around the axis of rotation on the photoreceptor, for example, the designated developing rollers have been swung one after another to make such a developing roller contact or adjoin the photoreceptor.
However, the aforesaid related-art color image forming apparatus requires a precision transfer mechanism for moving the developing rollers, space and time therefor. Consequently, the adoption of the system of moving the developing rollers on a color basis has constituted obstacles to reducing the cost and size as well as increasing the operating speed of such an apparatus.
As a method of solving the foregoing problems, it is conceivable to fixedly place the four color developing rollers at the predetermined positions around the photoreceptor by adopting a non-contact developing system in order not to necessitate moving the developing rollers. However, there are the following problems for making this method effectual.
A developing gap on the non-contact developing system is provided between developing rollers and a photoreceptor so as to carry out development. However, in case where the gap is too large, neat development is unavailable because toner is insufficiently transferred from the developing roller to the photoreceptor, whereas in case where the gap is too small, the toner is cause to transfer from the developing roller not in operation and there occurs a phenomenon of color mixture. Consequently, the problem is that high-quality development is difficult to carry out on the system.
As four of the developing units are needed to be disposed around the photoreceptor on the system above, the mounting angles of the developing units differ from each other and it is needed to contrive to prevent toner from being left over in a toner container in each developing unit. Moreover, as each developing roller is installed close to the photoreceptor, the developing devices must be arranged so that the photoreceptor is protected from being damaged when they are removed and replaced.
It is therefore an object of the invention to provide a color image forming apparatus on a non-contact developing system, capable of preventing toner in color being not developed from transferring to a photoreceptor even though the gap between developing rollers and the photoreceptor is decreased, using toner without waste and making developing devices removable and replaceable with safety.
In order to accomplish the object above, according to the invention, there is provided a color image forming apparatus, comprising:
a photoreceptor, on which a latent image is formed;
a plurality of toner carriers, each carries toner of a single color to be transferred onto the photoreceptor to make the latent image visible, each of the toner carriers fixed at a predetermined distance from a surface of the photoreceptor;
a first bias supplier, which supplies a first bias required to transfer the toner to the photoreceptor to one of the toner carriers which is subjected to a developing operation; and
a second bias supplier, which supplies a second bias which is not enough to transfer the toner to the photoreceptor to the other toner carriers which are not subjected to the developing operation.
In such an apparatus, even though the distance between the photoreceptor and the toner carriers is decreased, toner color being not developed is prevented from mixing with what is being developed, so that high-quality printing can be done.
Preferably, the first bias includes an AC component and a DC component, and the second bias is the DC component of the first bias.
In order to accomplish the object above, according to the invention, there is also provided a color image forming apparatus, comprising:
a photoreceptor, on which a latent image is formed;
a plurality of toner carriers, each carries toner of a single color to be transferred onto the photoreceptor to make the latent image visible, each of the toner carriers fixed at a predetermined distance from a surface of the photoreceptor;
a first bias supplier, which supplies a first bias having at least one first absolute value required to transfer the toner to the photoreceptor to one of the toner carriers which is subjected to a developing operation; and
a second bias supplier, which supplies a second bias having a second absolute value less than the first absolute value to the other toner carriers which are not subjected to the developing operation.
Preferably, the first bias includes an AC component and a DC component having the first absolute value.
Preferably, a plurality of first absolute values are provided according to colors of the toner.
In the above configurations, it is preferable that the toner is stored in a toner container disposed above each of the toner carriers. In this case, even in the system of disposing the toner carriers around the photoreceptor, the toner is never left in the toner containers without being supplied to the photoreceptor, so that the toner can be used up without waste.
In the above configurations, it is preferable that the apparatus further comprises: a plurality of developing devices, each including at least one of the toner carriers, each of the developing devices disposed at an installing position and movable from the installing position in a first direction; and a guide member, which guides each developing device such that a distance between the surface of the photoreceptor and each toner carrier is enlarged in a second direction substantially perpendicular to the first direction, in accordance with a movement of each developing device in the first direction.
Accordingly, the developing devices can be safely removed and replaced without damaging the photoreceptor.
In the above configurations, it is preferable that the photoreceptor is at least one of a plurality of photoreceptors provided in the apparatus.
According to the invention, it is thus possible to realize a high-quality color image forming apparatus on a non-contact developing system and to provide such a color image forming apparatus as is higher in operating speed and lower in cost and size than what utilizes the related-art developing-roller moving system.